Method of establishing a wireless multi-hop network

ABSTRACT

The invention describes a method of establishing a wireless multi-hop network (NW), in particular a ZigBee-type network, comprising a plurality of devices ( 0, 1, 2, . . . , 96 ) of a device arrangement (D), in which method devices ( 1, 2, 3, . . . , 96 ) establish a physical wireless connection to at least another device ( 0, 1, 11, 16, 17, 29, 30, 39, 48, 52, 53, 55, 64, 71, 73, 79, 88 ) of the network (NW) in a self-organizing process. In the self-organizing process a seeking device, intending to join the network, listens for beacon signals emitted by candidate parent devices already in the network comprising a network identifier and a device identifier of the emitting devices. Then, in a parent selection process, the seeking device selects a parent device from among the candidate parent devices, according to given selectionrules, based on the network identifiers (EPID), acceptance capabilities of the candidate parent devices, and link quality parameter values relating the device and the candidate parent devices. In this parent selection process application-level connection data (L, BI) of the seeking device and/or the candidate parent devices are applied. At least, the seeking device connects physically, and preferably also logically, via the selected parent device ( 0, 1, 11, 16, 17, 29, 30, 39, 48, 52, 53, 55, 64, 71, 73, 79, 88 ) to the network (NW). The invention further describes a device comprising a network inter-face for connecting a wireless multi-hop network (NW) according to this method.

FIELD OF THE INVENTION

The invention describes a method of establishing a wireless multi-hop,preferably mesh, network, in particular a ZigBee-type network,comprising a plurality of devices in a device arrangement. The inventionfurther describes a device comprising a network interface for connectinga wireless multi-hop network.

BACKGROUND OF THE INVENTION

Use of wireless networks is becoming widespread in business (commercial,industrial, institutional) and also consumer markets for the automaticcontrol of various device arrangements. Examples are building automationsystems, e.g. for lighting, heating & ventilation, safety, etc.,comprising devices such as light ballasts, switches, dimmers or othercontrol elements, daylight/occupancy sensors, actuators, meters etc. Useof wireless control makes the automation devices independent of themains power as control medium, thus allowing freedom of deviceplacement, since control of the devices is no longer dependent on powerwires and power outlets, and allowing greater device portability, atleast for those devices which may be battery-powered such as switchesand sensors. Typical examples for such wireless personal networks (WPAN)are ZigBee (IEEE 802.15.4), Bluetooth, HomeRF or Wi-Fi networks.

In many cases, the transmit range of the air interfaces between thedevices is smaller than the dimensions of the network, and it may benecessary, for an application-level connection, for a first device toestablish a network path (in the following also referred to as “route”)to a second device via a number of third devices which pass a messagefrom the first device to the second device. Such a network in which, onapplication-level, a sending device transmits a message to a receivingdevice using other devices as intermediate stations is referred to as a“multi-hop” network. Thereby, the network path between a sending and areceiving device may be established in a self-organizing manneraccording to rules of the relevant network standard. For it, physicalconnections—i.e. the fact of devices being directly in each others radiorange, especially in mesh networks—and logical connections—i.e. thespecial relationships between the nodes, especially in a tree-basedtopology—are used. The physical and logical connections between thedevices may be established in an initialisation process, for example,when a device joins the network. In another example, a network path maybe established when it is needed, in an ad-hoc manner, in particularwhen a previous network path breaks down, for example, owing to thefailure of an intermediate device in the path.

Currently, ZigBee is the only standard low-power, low-footprint WPANtechnology that allows self-organization to a large extent. ZigBeeoffers two ways of establishing the network topology: ‘DirectJoin’ andfree association, whereby in both cases a tree structure may beestablished, which is the preferred allocation for smaller networks, ora stochastic structure may be established which can be a favourablesolution for larger networks. The device at which the new device joinsis called the ‘parent device’ of the joined device (‘child device’).

DirectJoin method requires the user to pre-define the parent-childrelationships for all devices in the network. Besides placing a highburden on the user (especially in large networks with thousands ofnodes), this requires expert tools to measure radio range and/or expertknowledge on wireless propagation, and is associated with a highmaintenance effort when alterations or adaptations must be made to thenetwork. This is undesirable in the instable, multipath-susceptiblewireless environment.

The free association procedure allows a seeking device (either acompletely new one or one that had been in the network before) intendingto join the network, to select a parent node according to pre-definedcriteria. In the ZigBee standard, both reduced-function ZigBee EndDevices (also referred to as ‘ZED’ in the following) and ZigBee Routers(also referred to as ‘ZR’ in the following) attempt to select as aparent a router for which all the following conditions are true:

1. The router belongs to the network identified by a particular networkidentifier (‘ExtendedPANId parameter’ or ‘EPID’ in the ZigBee-standard);

2. The router is open to join requests and advertises capacity of thecorrect device type (ZigBee Router or ZigBee End Device);

3. The announced ‘update id’ is recent;

4. The link quality for frames received from this device is such that alink cost of at most 3 is produced. [ZigBee-2007, 053474r17, sec.3.6.1.4.1.1, p. 352, 1. 1 ff],

whereby, the link cost is calculated as

$\begin{matrix}{{C\{ 1 \}} = \{ \begin{matrix}{7,} \\{\min ( {7,{{round}( \frac{1}{p_{1}^{4}} )}} )}\end{matrix} } & (1)\end{matrix}$

where p₁ is the probability of packet delivery on the link 1 and can bearrived at by analyzing packet error rates and/or signal properties suchas Energy Detection and/or Signal-to-Noise ratio values; at ZigBeeimplementer's discretion.

5. If more than one device meets these four requirements, then,according to the ZigBee standard, the joining device shall select theparent with the smallest tree depth. [ZigBee-2007, 053474r17, sec.3.6.1.4.1.1, p. 352, 1.18 ff]. If more than one potential parent has thesmallest depth the device is free to choose from these.

On initial network formation, the devices will not yet have a goodestimate of the link cost since very few packets will have beensent/received by then. When the default value of 7 is taken for the linkcost (see equation (1)), all possible parents meet the requirements 1-4with a tie on the fourth requirement, which will effectively cause thenode to apply the fifth rule only, i.e. to choose the parent highest inthe network tree (with the lowest tree depth).

Therefore, the above free association method may result in networktopologies that are very dense in close proximity to the tree root—i.e.to the ZigBee PAN Coordinator (in the following also named ‘ZC’), whichforms the root of the network logical structure (esp. tree)—and mayresult in parent-child links of potentially poor quality (at least,higher quality links might be available). As a result, the performanceof the network may be less than optimal with regard to both delay andreliability of packet delivery. This is especially true for the networksincorporating ZEDs, which are compelled by the standard to communicatesolely via their parent.

Many ZED-like reduced-function, battery-operated devices are expected inthe ZigBee-like networks, because the strength of the wireless controllies in its independence of mains power. The control devices—switches,sensors, remote controllers (RC), etc., using radio mostly in reactionto user input or sensed environmental change—are expected to bebattery-operated ZEDs. Multiple devices may be controlled by them,including packet-loss sensitive and time-critical applications likelight switching, requiring—despite ZED's reduced functionality—certainnetwork performance level. Furthermore, it is desirable to maximizenetwork reliability, and thereby conserve ZED's energy, by keeping thenumber of re-transmissions associated with a command to a minimum.

Therefore, it is an object of the invention to provide a moreintelligent self-organization method for the formation of wirelessmulti-hop networks, in particular ZigBee-type networks that includeZEDs, and to provide a device comprising a network interface forconnecting a wireless multi-hop network according to this method.

SUMMARY OF THE INVENTION

To this end, the present invention describes a method of establishing awireless multi-hop network comprising a plurality of devices in a devicearrangement, in which method devices establish a physical wirelessconnection to at least another device of the network in aself-organizing process in which self-organizing process

-   -   a seeking device, intending to join the network, listens for        beacon signals emitted by candidate parent devices already in        the network, wherein a beacon signal comprises a network        identifier and a device identifier of the emitting device;    -   the seeking device selects a parent device from among the        candidate parent devices in a parent selection process,        according to given selection rules, based on the network        identifiers (EPID), acceptance capabilities of the candidate        parent devices, and link quality parameter values relating the        device and the candidate parent devices,

in which parent selection process further application-level connectiondata of the seeking device and/or the candidate parent devices areapplied;

-   -   and the seeking device connects physically, and preferably also        logically (“associates” is the term used in the        ZigBee-standard), via the selected parent device to the network.

In a ZigBee network the candidate parent devices are the ZRs (or a ZC)sending their ExtendedPANId parameter as a network identifier and IEEEaddress as device identifier in a so-called “beacon” signal as aresponse signal to a beacon request signal of a seeking device. Theacceptance capability is the capability of the candidate parent deviceto accept a device of a specific type, e.g. a ZED or a ZR, as a furtherchild, which information is also included in the beacon signal. The linkquality parameter value may be the link costs determined according toequation (1).

In contrast to the current standard, application-level connection dataare now taken into account in the parent selection process. Suchapplication-level connection data may be any data disclosing to whichother device a device is directly or indirectly bound or connected on anapplication-level, as well as how important this binding is, e.g.whether a switch is bound to a luminaire controlled by the switch, orwhether two or more luminaires are in the same functional group and arecontrolled by a common switch, or which of the two monitor stations asensor has to report to more frequently. Using this method, the closestbound device, i.e. the candidate parent device which has best linkquality and is bound to the seeking device, is preferably chosen as theparent device. As will be shown later, this approach leads, in a verysimple way, to an enormous improvement in the performance of a treestructure network established by a self-organizing method, since thenumber of hops used to establish the network path may be reducedconsiderably.

An appropriate device comprises a network interface for connecting sucha wireless multi-hop network, which network interface comprises

-   -   a listening unit which listens, when the device is intending to        join the network, for beacon signals emitted by candidate parent        devices already in the network which beacon signals comprise        network identifier and device identifier of the candidate parent        devices,    -   a parent selection unit for selecting a parent device from among        the candidate parent devices, according to given selection        rules, based on the network identifiers (EPID), acceptance        capabilities of the candidate parent devices, and link quality        parameter values for the link between the device and the        candidate parent devices,

which parent selection unit is realised such that it also appliesapplication-level connection data of the device and/or the candidateparent devices in a parent device selection process,

-   -   and a connection unit for establishing a physical, and        preferably also a logical, connection in the network between the        device and the selected parent device.

The invention is preferably used for networks of the ZigBee-type but mayalso be used for other similar multi-hop networks that are able toestablish physical, and preferably also logical, connections between thenetwork nodes in a self-organizing process.

The dependent claims and the subsequent description discloseparticularly advantageous embodiments and features of the invention,whereby, in particular, the device according to the invention may befurther developed according to the dependent method claims.

In a preferred embodiment of the invention, a seeking device sends arequest signal to devices already in the network. A number of candidateparent devices that receive the request signal may then each emit abeacon signal as a response. Such a request signal for scanning forpossible parent devices may be broadcast by a scan unit or module of thenetwork interface. This method is simpler and less energy consumingsince the candidate parent devices need not broadcast a beacon signalcontinuously, but only when a device intends to join the network.

If a seeking device receives a beacon signal from a device with thecorrect network identifier, the seeking device can then request thatemitting candidate parent device to accept it as a child. In order toreduce data traffic, the beacon signals of a candidate parent devicepreferably already comprise an acceptance capability for that devicetype (in ZigBee: ZR or ZED).

Furthermore, the beacon signal may already comprise tree depthinformation of the emitting candidate parent device, indicating thenumber of nodes between the emitting candidate parent device and a rootof the network tree structure, for example, the ZigBee-Coordinator in aZigBee-network. Such tree depth information may then also be taken intoaccount in the parent selection process. For example, in the case of twoequally suitable candidate parent devices, in consideration of theapplication-level connection data, the candidate parent devices with thesmaller tree depth may be chosen in a manner similar to the currentZigBee-standard.

As mentioned above, the application-level connection data may be anydata that define to which other device a device is directly orindirectly bound or connected to on an application-level, or theimportance of this binding. On the basis of this data, a seeking devicecan favourably select as parent device a device that, on anapplication-level, acts as a controller for that seeking device or thatis, on an application-level, controlled by that seeking device. Forexample, in a ZigBee-network a switch (ZED or ZR) may choose one of theZR luminaires which are controlled by it.

This requires devices to know the other devices with which it willcommunicate on the application layer. This can be easily achieved, forinstance, if the application-level connection data comprises acollection, for example a simple list, of device identifiers of otherdevices to which the devices should be connected on an applicationlevel. This identifier collection or list (referred to in the followingas “binding table”) may be stored in an appropriate memory of thenetwork interface.

In this way, for example, a controller could select as parent one of thedevices it is bound to by comparing the device identifiers in the beaconsignal and stored in the binding table. For this purpose, the unique64-bit long addresses of the devices in a ZigBee-network are preferablyused as device identifiers.

In extension to the rules defined for the controller ZEDs in theapproach described above, other devices to be controlled by the samecontroller, could, if possible, especially if they are ZEDs themselves,select as their parent the parent of the controller in order to minimizethe number of hops.

The devices of the device arrangement may be grouped into functionalgroups and the application-level connection data might comprise groupmembership data. By also using common group membership as a criterionfor parent selection as well, a reduced number of hops inapplication-level connection can be achieved. However, this requiresgroup membership configuration prior to association, and knowledge ofother group members, which may be supplied in the form of a simple groupidentifier in the beacon signal.

The bindings and/or functional groups are preferably configured in theusual commissioning phase, so that the application-level connection dataare available for parent selection for a device intending to join anoperational network. They may also be statically pre-configured, e.g. bydevice (set) vendor or a system integrator, e.g. established via Out OfBand methods.

In a preferred embodiment of a ZigBee-network, the application-levelconnection data comprise binding data in an Application Support Sublayer(APS). Alternatively, the applications could be provided with thisinformation directly, i.e. application-level connection data aredirectly associated to application objects of the devices, without usingthe APS binding mechanism. In the following therefore, for simplicity,any logical connection on application-level between a source device anda destination device may be referred to as “binding” and the respectivedevices may be referred as “bound devices”, regardless of whether theAPS binding mechanism is used or not.

As explained above, a seeking device should preferably select a parentfrom among the candidate parent devices to which it is bound. However,particularly when a new network is being established, those bounddevices may not yet be in the network when the seeking device tries tojoin. Thus, in a preferred embodiment a seeking device sends a newrequest signal if, subsequent to the first request signal, it does notreceive a response signal from a candidate parent device to which therequesting device should be connected on an application level, accordingto the application-level connection data. In this way, the seekingdevice, especially a ZED, could postpone, preferably within a given timelimit, the association until at least N of the bound devices (N being 1or more) are discoverable by the initial scan, since the seeking devices“knows” that the bound devices will join the network. This will allowfor formation of even more efficient topologies, while not delaying thenetwork formation. The information about which bound devices located inthe neighborhood, if any, may e.g. be collected during the commissioningphase and stored. For example, the beacon signals of the potentialparents collected during the commissioning phase may be stored with theapplication-level connection data in the memory of the seeking device.

When a seeking device joins a network, in particular a network which isin the process of being established, the seeking device may find a boundparent device which is the closest at that time. However, other possiblesuitable parent devices that are even closer might have started laterand cannot therefore be found by that seeking device during association.To remedy this situation, in a preferred embodiment of the invention, adevice waits for a given delay time after an initial start-up beforeseeking a parent device. For example, the seeking devices could bydefault delay their initial association procedure, which might involvebroadcasting the request signal, so that they can select a parent devicefrom a larger number of available, already associated, devices. Thiscould be realized e.g. as a default sleeping period after power-upand/or reset; preferably only in operational mode and not incommissioning mode. As opposed to simply extending ScanDuration in thecurrent ZigBee-standard, this method does not increase the initialenergy consumption by the seeking devices.

The ZigBee specification allows a Full Functional Device to associateeither as a ZR, or —if that not possible owing to potential parents'addressing space or memory entries for children being exhausted—toassociate as a ZED. For a Full Functional Device that cannot find aparent device according to the standard ZigBee approach as a ZR (esp.due to unfulfilled criteria 2 and 4), it may be more beneficial tobecome a ZED child of this nearby bound router, instead of associatingas a ZR to a more remote bound or unbound router. Therefore, in afurther preferred embodiment of the invention, a seeking device withfull function device capability may connect physically as a reducedfunction device to another router device having no further acceptancecapability for a network router device, if that network router device isthe most suitable candidate parent device, according to theapplication-level connection data. This approach will further improvethe performance of networks that employ tree routing.

If many candidate parent devices fulfilling the criterion of one of theabove methods are available, the seeking device could select the onewith the strongest signal. Preferably, according to criterion 4 of thestandard ZigBee association process explained above, the link qualityparameter values relating the seeking device and the candidate parentdevices are applied in the parent device selection process. In this way,the seeking device increases the probability of delivering theapplication message to at least one bound device and reduces the numberof network hops required for this delivery.

In another embodiment, to select a parent from among many bound devicesin its vicinity fulfilling all of the above-discussed network andapplication criteria, the seeking device could use any furtherapplication data in the form of an indication of how often theapplication messages will be sent to particular bound devices, such asthe maximum or minimum update interval set by the application. Theseeking device may then select as parent device the device with which itwill most often communicate, thus optimizing for the biggest contributorto its traffic flow, by reducing this communication to one hop.

In yet another embodiment, if priorities and/or quality of serviceindicators are defined, e.g. attributing importance levels for selectedcommands and/or parameters, to particular logical connections, ports orsockets or complete applications, whether assigned by devicemanufacturer or in the process of commissioning, the seeking device mayselect from among the bound devices for its parent that device that hasthe highest priority and/or the most strict QoS (quality of service)requirements (e.g. minimum delay, maximum reliability). To this end,regarding a given application-level connection between the seekingdevice and a device which is bound to the seeking device on anapplication level, the application-level connection data may preferablycomprise, a quality of service indicator and/or priority data and/ordata regarding an estimated application-level message frequency betweenthe seeking device and the corresponding bound device. For example, inZigBee such indicators could be included in the seeking device's bindingtable.

The applicability of the method according to the invention could belimited to only some device types or device roles. Some devices, despitebeing end devices and/or battery powered, may not require this method.For example the central light switch for an entire building may not bein the immediate vicinity of any of its bound devices and/or may notrequire fast response times. The information collected in thecommissioning phase, if used, can be used to determine this case. Asanother example, a portable remote controller may not be permanentlybound to any device; it may benefit from associating quickly to thefirst device in its vicinity with reasonable link costs.

When a seeking device determines that none of the bound devices have thecapacity to accept new children, either because they are not routers, ortheir child capacity is exhausted, or because they don't fulfill theother parent selection criteria, the seeking device may revert to thestandard procedure.

Preferably, a device already joined in the network may also exchange itsparent device if it receives new application-level connection data. Forexample, if after network formation (according to the standard method orone of the methods disclosed above), the application-layer messages arenot reliably delivered, the seeking device could decide to replace itsparent. In a ZigBee network, this could be triggered, for instance, by alack of the end-to-end application level responses, APS level ACKs, theMAC level ACKs (from the parent), and/or changing link costs. Proactivesubstitution of the parent may be required, if the binding informationbecomes available only after joining the network, e.g. if nocommissioning phase is used.

The same procedures as described above can be used by a device thattries to re-join the network; e.g. an orphaned device—i.e. a device thatlost the connection to its parent, e.g. due to parent removal orwireless propagation change. Relying on the link cost and performancedata collected previously when joined to the network, the device canchoose the new parent, taking the application-layer data into account.

Evidently, the parent device could also use the application-layer databefore accepting an association request or join request from a child.For example, when the parent can only accept one child, and it receivesthe association request from two potential children at essentially thesame time, of which only one is bound to the parent, the parent routermay prefer the bound device.

Preferably, router devices may apply the above-described methods toselect their physical connections to other neighbour devices than theparent. The memory of embedded devices, whether ZR or ZED, is usuallylimited, allowing for storing only pre-defined number of neighbors,children and routers. In dense networks, each router may have a numberof physical connections that largely exceeds the space available in theneighbor table. Thus when populating the neighbor table, instead ofrelying solely on the discovery order and/or link cost indicators, therouters could also use the application-layer connection data, such ase.g. the existence of application-layer connection or its priority.

Other objects and features of the present invention will become apparentfrom the following detailed descriptions considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are designed solely for the purposes of illustration and not asa definition of the limits of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic representation of a ZigBee protocol stack forexplanation of the parent selection process according to the currentstandard;

FIG. 2 shows a flow chart of the association process of an unconnectedseeking device according to the current standard;

FIG. 3 shows a schematic representation of an embodiment of a devicearrangement comprising several functional groups of devices in differentrooms;

FIG. 4 shows an exemplary ZigBee association graph for the devicearrangement of FIG. 3 for a free association tree topology according tothe current standard;

FIG. 5 shows a schematic representation of a ZigBee protocol stack forexplanation of the parent selection process according to an embodimentof the invention;

FIG. 6 shows a flow chart of the association process of an unconnectedseeking device according to an embodiment of the invention;

FIG. 7 shows an exemplary ZigBee association graph for the devicearrangement of FIG. 3 for a free association tree topology according toan embodiment of the invention.

In the drawings, like numbers refer to like objects throughout. Objectsin the diagrams are not necessarily drawn to scale.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, without restricting the invention in any way, it isassumed that the network is a ZigBee-type network since ZigBee iscurrently the most common wireless mesh networking standard.Nevertheless, the invention may also be used in similar multihopnetworks.

FIG. 1 shows a typical protocol stack of a ZigBee protocol. ZigBeedevices are required to conform to the IEEE 802.15.4-2003 Low-RateWireless Personal Area Network (WPAN) standard. The standard specifiesthe lower protocol layers—the Physical layer PHY, and the Medium AccessControl MAC portion of the data link layer. This standard specifiesoperation in the unlicensed 2.4 GHz, 915 MHz and 868 MHz ISM bands. TheNetwork layer NWK is responsible for addressing and packet routing,providing end-to-end data transmission services to the higher layers ofthe protocol stack which are part of the Application Framework AF. TheApplication Framework AF includes an Application Support Sublayer APS, anumber (up to 240 per ZigBee device) of Application Objects AO₁, . . . ,AO₂₄₀ and a part of a ZigBee Device Object ZDO. The Application SupportSublayer APS is, for example, responsible for binding management. TheApplication Objects AO₁, . . . , AO₂₄₀ are implemented by a developer ofthe device which uses the ZigBee interface, and may follow a proprietaryor standardized ZigBee application profile, such as e.g. Home Automation(HA), Commercial Building Automation (CBA) or Smart Energy (ZSE)profile. The ZigBee Device Object ZDO manages the ZigBee device andis—among others—responsible for the network selection, for theinitialisation of the ZigBee Layers (with the exception of theApplication Objects AO₁, . . . , AO₂₄₀), the definition of the actualfunction of the ZigBee device (ZED, ZR or ZC), the discovery of otherdevices in the network and the initiation of sending binding requests. ASecurity Service Provider SEC is responsible for exchanging andmaintaining security credentials, as well as security operation, such asencryption and authentication.

Referring to FIG. 2, it will be roughly explained how an unconnectedZigBee device may join an existing ZigBee network according to thecurrent ZigBee standard. Thereby, the still unconnected seeking deviceintending to join the network will scan for potential parent devices bybroadcasting a request signal, listening for beacon signals from deviceswithin range, and determining the link cost for each responding device.The scan may be performed by a listening unit LU of the Medium AccessControl layer MAC (see FIG. 1), and a link cost calculation unit LC maycalculate the link costs according to equation (1) above.

The data from the beacon signal of each discovered potential parentdevice, in particular the network identifier EPID, the 16-bit and the64-bit addresses, the child acceptance capability and the tree depth,which are all transmitted with the beacon signal, as well as thecalculated link costs, are transferred to the Network layer NWK whichcollects the data of the beacon signals. In the next step, a parentselection unit PS of the Network layer NWK selects a parent device fromamong the candidate parent devices, according to the ZigBee standardselection criteria 1 to 4 explained above, based on the data receivedwith the beacons and the link costs. To select only parent devices ofthe correct network, the parent selection unit PS is provided with thenetwork identifier EPID of the selected network from a network selectionunit NS of the ZigBee Device Object ZDO.

If only one candidate parent device fulfilling the criteria is found, aconnecting unit CU of the Medium Access Control layer MAC will initiatea physical connection (association) to that selected parent device, i.e.the seeking device will join the network with the selected EPID at theselected parent device. If the number of candidate parent devicesfulfilling the criteria is greater than one, the parent selection unitwill choose the parent device based on the tree depth (criterion 5,mentioned above). The details of such an association process will beknown to those skilled in the art, so that no further detailedexplanation is needed at this point.

It should be noted that the listening unit LU, the link cost calculationunit LC, the parent selection unit PS, the binding management unit BM,the network selection unit NS and the connection unit CU are allrealised as software modules or algorithms running on a processor of theZigBee interface of the ZigBee device, and are indicated in the specificZigBee layers in the protocol stack of FIG. 1 for an easierunderstanding.

It can be shown that this default parent selection mechanism as proposedby the ZigBee specification leads to suboptimal network topologies and,therefore also to suboptimal network performance, especially if ZEDs areinvolved.

A device arrangement D in a typical office environment is taken as abenchmark and is shown in FIG. 3. The office environment consists of tenrooms and a corridor leading to each of them. The device arrangement Dis in this case a lighting arrangement, but it will be noted at thispoint that the invention is not in any way restricted to lightingsystems.

Each room is equipped with six lamps being ZigBee Routers 7-12, 16-21,25-30, 34-39, 43-48, 52-57, 61-66, 70-75, 79-84, 88-93, controlled by(i.e. bound to) one switch per room being a ZED 15, 24, 33, 42, 51, 60,69, 78, 87, 96. In each of the rooms an unbound ZED power outlet 14, 23,32, 41, 50, 59, 68, 77, 86, 95 and an unbound ZED light sensor 13, 22,31, 40, 49, 58, 67, 76, 85, 94 are installed. The lamps 0, . . . , 5 inthe corridor are controlled by the floor switch 6. All these devices 0,1, 2, . . . , 96 are grouped into functional groups R₁, R₂, . . . , R₁₀,F according to the room in which the devices are installed. Each device0, 1, 2, . . . , 96 is represented by a simple circle.

For this device arrangement D, an initial network formation is performedaccording to the standard ZigBee procedure as explained above. It isassumed that reliable link quality parameter values are not yetavailable (e.g. due to the fact of implementing an integrating function,so that effectively a default value of 7 for the ZigBee link costs istaken), so that parents are selected on the basis of their position(depth) in the network tree. It is further assumed that the device 0 wasthe first device and acts as the ZigBee coordinator. An exemplary ZigBeeassociation graph for a free association topology achieved with thismethod is shown in FIG. 4. The ZigBee Cskip parameters for a logicaltree formation were set to: Cm=30, Rm=12, Lm=4. Logical connections witha tree depth of “1” are indicated by dashed lines; connections with atree depth of “2” are indicated by solid lines, whereby the linesoriginate in the centre of the circle indicating the parent device andterminate at the perimeter of the circle indicating a child device. Ascan be seen, the node closer to the root (measured in hops along thelogical tree structure) is the parent; the node connected to this nodeis its child.

The performance of this network has been measured using the networksimulator ‘NS-2’. Thereby, the worst-case scenario was simulated: alleleven switches are toggled at exactly the same moment and each of themsends the message in unicast to all the six associated lamps, resultingin a storm of sixty-six messages. The experiment was repeated 10,000times, to obtain statistically relevant data. The performance resultsobtained in the simulation for the association graph shown in FIG. 4 arepresented in the following table:

TABLE 1 Simulation results for a network according to the associationgraph of FIG. 4 average fail rate 23.0% minimum fail rate 0.2% maximumfail rate 93.0% average delay (ms) 304 minimum delay (ms) 6.6 maximumdelay (ms) 528 delay <200 ms 22.5%

As can be seen from the table, on average 23% of the commands did notreach their destination. This poor performance of a free associationtopology according to the default ZigBee algorithm is caused by long andtherefore weak links, resulting in packets getting lost in thecollisions due to the hidden node problem. Furthermore, a multi-hop pathconsisting of a number of weak links increases collision probability forthe packet.

To overcome this weakness of the ZigBee standard, the ZigBee devices arerealized according to the invention, so that application-levelconnection information such as binding information and/or groupinginformation are taken into consideration during the parent selectionprocess.

The principle of this modification of the ZigBee standard will beexplained with the aid of FIG. 5 and FIG. 6, whereby FIG. 5 is agraphical representation of the ZigBee protocol stack as shown in FIG.1, and FIG. 6 shows a flow chart of the association process according toan embodiment of the invention.

According to steps I and II of the flow chart, the unconnected device isprovided with application-level connection data. This data may begenerated in the usual way in a commissioning process, in which thedevice identifiers are collected and grouped and, as part of the setup,configuration of application logic is performed, i.e. logicalconnections are configured to establish a control relationship betweendevices based on properties such as type and location. For example, in alighting system, it can be specified which switch should control whichlamp(s). The configuration of the application logic may be performedusing a central commissioning system which transmits the data to thedevices via the ZigBee interfaces.

In step III, the device will search for potential parent devices in theusual way by broadcasting the request signal, receiving the beaconsignals and determining the link cost for each responding device, usingthe listening unit LU and the link cost calculation unit LC as explainedin connection with FIG. 1 above. In step IV, the data of the beacon ofeach discovered potential parent device and the calculated link cost aretransferred to the Network layer NWK for collecting the data.

Also, the network selection unit NS of the ZigBee Device Object ZDOtransfers the network identifier EPID of the selected network to theparent selection unit PS of the Network layer NWK in the usual way.

In contrast to the current standard, the parent selection unit PS is nowalso provided with application-level connection data or bindinginformation from the binding management unit BM (see FIG. 5). Thisbinding information is available in form of a list L of the 64-bit longaddresses of the other devices to which the device is bound. Similarbinding information BI may be also provided from the applicationobjects, here as an example from the application object AO1. Normally,the binding management unit BM uses the binding information, whichcomprises the list L, only to establish the application-layer connectionto a destination device when the device itself is already in thenetwork. Using the method according to the invention, the list can nowbe used to advantage by the parent selection unit PS during the parentselection process. According to step V, the potential parent devices areonly selected according to criteria 1-4 mentioned above, from suchcandidate parent devices to which the device is bound. This can, forexample, be ensured by simply comparing the 64-bit addresses in thebeacons with the list L from the binding management unit. In this waythe bound device with the best link cost will be selected as parentdevice.

In step VI a check is performed how many potential parent devices havebeen found in step V. If only one potential parent device has beenfound, the connecting unit CU of the Medium Access Control layer MACwill initiate an association to this selected parent device, and thedevice will join the network at the selected parent device in step XII.

If the number of potential parent devices is greater than one,additional application-layer data is considered by the parent selectionunit, namely the frequency of the reporting. The frequency of reportingcan be obtained by reading the sender endpoint and cluster identifierper the destination 64-bit address from the binding table and thenasking the relevant application object. The parent selection unit willthen choose, in step VII, the device with the highest estimatedreporting frequency, i.e. the potential parent device with which theseeking device will probably most often communicate. If more than onepotential parent device has the same highest reporting frequency, whichis shown in step VIII, the parent device with the highest priority ischosen. The priority may be defined in the logic configuration processand laid down in the binding table. If more than two devices share thehighest priority, the bound device may be selected on the basis of thetree depth (steps X and XI).

If, in step V, no bound device is detected among the devices from whichthe beacons are received and the list L was not empty, the process—aftera waiting time (step XV)—returns to step III and the seeking devicescans for other devices again, since the seeking device ‘knows’ thatthere may be other devices to which it is bound. To prevent the devicefrom dead-lock in the association procedure, the return to step III isassociated with a timeout criterion, for example a given maximum time ora maximum number of scan rounds. If, in step XIV, the timeout criterionis fulfilled, the seeking device follows the usual parent selectingprocedure according to the current ZigBee standard in steps XVI, XVII,and possibly XI.

When the parent device is selected, the network is joined via theselected parent device in steps XII and XIII.

FIG. 7 shows an exemplary ZigBee association graph for a freeassociation topology in the same office environment as in FIG. 4, butnow achieved with the method according to the invention. In this method,as explained above, from potential parents the bound device with thebest link cost is chosen. Here, the logical connections with a treedepth of “1” are indicated by dashed lines, the connections with a treedepth of “2” are indicated by dashed-dotted lines, and the connectionswith the tree depth “3” are in solid lines. The same Cskip parameters asfor FIG. 4 were used: Cm=30, Rm=12, Lm=4.

The performance of this network is simulated using the network simulatorNS-2, again for the worst case scenario that all eleven switches aretoggled at exactly the same moment and each of them sends the message inunicast to all the six bound lamps, resulting in a storm of sixty-sixmessages. The performance results for the association graph shown inFIG. 7 are presented in the following table:

TABLE 2 Simulation results for a network according to the associationgraph of FIG. 7 average fail rate 2.01% minimum fail rate 0.0% maximumfail rate 62.6% average delay (ms) 136 minimum delay (ms) 3.3 maximumdelay (ms) 345 delay <200 ms 74.8%

As can be seen from this table in comparison to the results listed inTable 1 for the association graph of FIG. 4, the method offers atremendous improvement as compared to the benchmark, in terms of delayby a factor of more than 2, from 304 ms to 136 ms and in terms ofreliability, since the failure rate is reduced by more than 90%. Itshould be noted that the worst-case scenario was simulated here withsixty-six messages sent in a dense network at exactly the same time.Obviously, considerably better results will be obtained in anon-worst-case situation.

Although the present invention has been disclosed in the form ofpreferred embodiments and variations thereon, it will be understood thatnumerous additional modifications and variations could be made theretowithout departing from the scope of the invention. In particular, itshould be mentioned that the steps VII to X in the process according toFIG. 6 are optionally. For the sake of clarity, it is to be understoodthat the use of “a” or “an” throughout this application does not excludea plurality, and “comprising” does not exclude other steps or elements.A “unit” or “module” can comprise a number of units or modules, unlessotherwise stated.

1. A method of establishing a wireless multi-hop network (NW), inparticular a ZigBee-type network, comprising a plurality of devices (0,1, 2, . . . , 96) of a device arrangement (D), in which method devices(1, 2, 3, . . . , 96) establish a physical wireless connection to atleast another device (0, 1, 11, 16, 17, 29, 30, 39, 48, 52, 53, 55, 64,71, 73, 79, 88) of the network (NW) in a self-organizing process inwhich self-organizing process a seeking device, intending to join thenetwork, listens for beacon signals emitted by candidate parent devicesalready in the network comprising a network identifier and a deviceidentifier of the emitting devices, in a parent selection process, theseeking device selects a parent device from among the candidate parentdevices, according to given selection rules, based on the networkidentifiers (EPID), acceptance capabilities of the candidate parentdevices, and link quality parameter values relating the seeking deviceand the candidate parent devices, in which parent selection processapplication-level connection data (L, BI) of the seeking device and/orthe candidate parent devices are applied, and the seeking deviceconnects physically via the selected parent device (0, 1, 11, 16, 17,29, 30, 39, 48, 52, 53, 55, 64, 71, 73, 79, 88) to the network (NW). 2.A method according to claim 1, wherein a seeking device sends a requestsignal to devices already in the network (NW) and a number of candidateparent devices that receive the request signal each emit a beaconsignal.
 3. A method according to claim 1, wherein the beacon signalscomprise an acceptance capability of the respective candidate parentdevice.
 4. A method according to claim 1, wherein the beacon signalcomprises tree depth information of the emitting candidate parentdevice, which tree depth information is applied in the parent deviceselection process.
 5. A method according to claim 1, wherein theapplication-level connection data (L, BI) comprise a collection (L) ofdevice identifiers of other devices to which the device should beconnected on an application level.
 6. A method according to claim 1,wherein a seeking device selects as parent device a device that, on anapplication-level, acts as a controller for that seeking device or thatis, on an application-level, controlled by that seeking device.
 7. Amethod according to claim 1 wherein the devices (0, 1, 2, . . . , 96) ofthe device arrangement (D) are grouped into functional groups (R1, R2, .. . , R10, F) and the application-level connection data (L, BI)comprises group membership data.
 8. A method according to claim 1wherein the application-level connection data are configured duringcommissioning of the device arrangement.
 9. A method according to claim1, wherein application-level connection data comprise binding data (L)in an Application Support Sublayer (APS) and/or application-levelconnection data (BI) are associated to application objects (AO 1, . . ., AO 240) of the devices.
 10. A method according to claim 2, wherein aseeking device sends a new request signal if, subsequent to the firstrequest signal, it does not receive a response signal from a candidateparent device to which the requesting device should be connected to onan application level, according to the application-level connectiondata.
 11. A method according to claim 1, wherein a seeking device waitsfor a given delay time after an initial start-up before seeking a parentdevice.
 12. A method according to claim 1, wherein a seeking device withnetwork router device capability connects physically as a network enddevice to another router device having no further acceptance capabilityfor a network router device, if that network router device is the mostsuitable candidate parent device, according to the application-levelconnection data.
 13. A method according to claim 1, wherein theapplication-level connection data comprise, regarding to a givenapplication-level connection between the seeking device and a devicewhich is bound to the seeking device on an application level, a qualityof service indicator and/or priority data and/or data regarding anestimated application-level message frequency between the seeking deviceand the respective bound device.
 14. A method according to claim 1,wherein a device already joined in the network may change its parentdevice if it receives application-level connection data.
 15. A devicecomprising a network interface for connecting a wireless multi-hopnetwork (NW), which network comprises a plurality of devices (0, 1, 2,96) of a device arrangement (D) and in which network devices (1, 2, 3, .. . , 96) establish a physical wireless connection to at least one otherdevice (0, 1, 11, 16, 17, 29, 30, 39, 48, 52, 53, 55, 64, 71, 73, 79,88) of the network (NW) in a self-organizing process, which networkinterface comprises a listening unit which listens, if the device isintending to join the network, for beacon signals emitted by candidateparent devices already in the network comprising a network identifierand a device identifier of the candidate parent devices, a parentselection unit for selecting a parent device from among the candidateparent devices, according to given selection rules, based on the networkidentifiers (EPID), acceptance capabilities of the candidate parentdevices, and link quality parameter values relating the device and thecandidate parent devices, which parent selection unit is realised suchthat it applies application-level connection data of the device and/orthe candidate parent devices in a parent device selection process, and aconnection unit for establishing a physical connection in the networkbetween the device and the selected parent device (0, 1, 11, 16, 17, 29,30, 39, 48, 52, 53, 55, 64, 71, 73, 79, 88).